1. Field of the Invention
The invention generally relates to a liquid crystal lens, and more particularly to a liquid crystal lens having a preferable response speed suitable for operation under a low temperature environment.
2. Description of Related Art
Nowadays, camera module technologies are advancing toward a direction of superior image quality having millions of pixels and a high resolution packaged in a small volume. This has brought new applications for camera modules in mobile phones and increased the desire for consumer purchases. Moreover, among the functions which are gradually becoming commonplace in a mobile phone camera, the foremost in importance would be the auto-focus function. Generally speaking, if the camera resolution is above 1.3 million pixels, the pixel area of the image detector is reduced. Moreover, an image captured by a typical fixed-focus camera lens does not have sufficient depth of field for both far and short distances at the same time, and therefore the camera must rely on the auto-focus function to obtain a clear image quality. Since the mobile camera must be compact and thin, auto-focus on the mobile camera cannot be implemented by the conventional motor driven lens module for a typical digital camera, but must employ a more miniature and compact actuator device to achieve the purpose.
Actuator devices of mobile camera lens modules that are currently applied include the voice coil motor (VCM), the piezoelectric actuator, and the miniature step motor. In addition, the liquid lens may be adopted, which employs a voltage control to alter surface tension so as to change the lens curvature and the focusing characteristics. Moreover, the liquid crystal lens may also be adopted, which employs a voltage control to alter electric field distribution so as to change the refractive index of the lens and the focusing characteristics. However, each of these implementations for achieving auto-focus has its own drawbacks. For example, when adopting the mechanical actuator devices, the material cost, assembly precision, and anti-shock characteristics are keys. On the other hand, when adopting the static liquid lens, then the manufacturing techniques and the optical imaging quality are major challenges. In addition, liquid crystal lenses have a slow response speed and are not suitable for a low temperature operating environment. Therefore, an important research and development focus is finding a preferable technique which is adaptable on the mobile phone camera module for implementing auto-focus.
Over the years, numerous documents have disclosed various types of liquid crystal optical device techniques, in which the optical characteristic of the liquid crystal material with its refractive index changing relative to an externally applied voltage is utilized to alter the focal distance of the lens, and these techniques, are being applied in various product ranges. For example, U.S. Pat. No. 6,864,951 discloses combining inhomogeneous polymer dispersed liquid crystals (PDLC) with ultraviolet (UV) light illumination so as to form droplets of uneven size from liquid crystal molecules. Moreover, the optical focusing characteristic is continuously modulated by variation of the externally applied voltage. Additionally, an article in the international journal Applied Optics (Vol. 43, No. 35, p. 6407, December, 2004) discloses designing the electrodes at the other side of a glass which is far thicker than the liquid crystal dielectric layer. By applying a large voltage and penetrating the potential distribution through the glass, a continuously curved surface distribution is formed in the liquid crystal dielectric layer. Accordingly, the liquid crystal refractive index has a continuous distribution much like a lens with a continuous phase distribution, and the liquid crystal lens has good focusing characteristic and a simple device structure. However, the driving voltage needs to be 100 V, and the response speed needs to be increased. In previously described conventional liquid crystal optical devices such as U.S. Pat. Nos. 6,577,376 and 6,690,500, since the diffraction device designs employ strip shape blocks or Fresnel lens, each diffractive step generated by diffraction lowers the overall utilization efficiency of the light beam. On the other hand, U.S. Pat. No. 6,864,951 discloses a PDLC framework, although due to the strict nature of the UV light exposure process, and other factors such as a low medium transmittance from diffraction and a high device driving voltage, the response speed problem remains an issue.